Earth and Space Science / Unit 4: Earthquakes and Volcanoes
4.6 Volcanoes and Their Types
Notes
A volcano is a vent where magma, ash and gas reach the surface.
- Shield volcano: broad, gentle slopes, runny basaltic lava, mild eruptions (Mauna Loa).
- Composite (stratovolcano): steep cone of alternating lava and ash layers, thick sticky lava, explosive (Mount St. Helens, Fuji).
- Cinder cone: small, steep, made of loose rock fragments.
- Magma that is rich in silica is thick and traps gas, causing explosive eruptions.
- Volcanoes form at subduction zones, divergent boundaries and hot spots.
Practice
Which volcano type has gentle slopes and runny lava?
Low-silica basaltic lava flows far.
Why are eruptions of high-silica magma often explosive?
Thick magma blocks gas escape until pressure bursts out.
Mount St. Helens is a:
It is a steep explosive volcano at a subduction zone.
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